The fundamental operation of hydraulically controlled variable camshaft timing systems (hereinafter referred to as "VCT systems") is well known. For example, U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position of a camshaft relative to a crankshaft.
U.S. Pat. No. 5,107,804 also further describes a VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing, the vane being oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft.
Generally, the control system for the VCT systems of U.S. Pat. Nos. 5,002,023 and 5,107,804 utilize a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders or lobes is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool may occur in response to an increase or decrease in control hydraulic pressure on an end of the spool and the relationship between the hydraulic force on such end and an oppositely direct mechanical force on the other end.
Although the many known VCT systems have been proven to perform satisfactorily, further improvements on the operation of VCT systems of the type described herein are desired. For example, an hydraulically operated VCT system relies on the presence of pressurized engine lubricating oil or other hydraulic fluid within the VCT system to function properly and predictably. This condition is achieved during normal engine operation, when engine lubricating oil is used as the hydraulic fluid within the VCT system, since adequate pressurization of the engine lubricating oil inherently results from the operation of the engine. However, when an automotive engine is shut off, the pressure of the engine lubricating oil soon drops, and the hydraulic fluid within a VCT system of the aforesaid type will normally drain back to the engine crankcase introducing unwanted air into the VCT system.
It will be appreciated that during engine start-up the unwanted air is distributed throughout the VCT system thereby causing excessive oscillation of the hydraulic cylinders or lobes of the VCT system which in turn causes noise in the VCT system or the associated valve train until such time as the air is displaced with hydraulic fluid. Consequently, there is a significant need for an improved VCT system that overcomes the problems associated with hydraulic fluid draining from the VCT system when the engine is not running and with the introduction of air into the hydraulically operated VCT system. Furthermore, there is a significant need for an improved VCT system that prevents hydraulic fluid from draining from the VCT system when the engine is not running and removes air from the VCT system when the engine is running to facilitate the quiet operation of the VCT system and prevent unwanted noise.
In addition to the foregoing, there is a significant need to operate the VCT system to a controlled known position when there is zero hydraulic pressure in the hydraulic control system until the VCT system obtains sufficient hydraulic fluid pressure. As used herein the term "sufficient hydraulic fluid pressure" refers to the lowest possible hydraulic fluid pressure at which the engine will operate. Operation of the VCT system in a known position until the pressure of the hydraulic fluid within the VCT system is adequately high ensures normal, quiet operation of the VCT system during all phases of engine operation including periods when the engine has not developed full hydraulic fluid pressure such as during engine start-up.